Control of reduction pot lines



March 29, 1960 R. J. COOPER CONTROL OF REDUCTION POT LINES 4Sheets-Sheet 1 Filed Jan. 10, 1957 INVENTOR Fade/7 I (ac oer March29,1960 R. J. COOPER CONTROL OF REDUCTION POT LINES 4 Sheets-Sheet 2Filed Jan. 10, 1957 March 29, 1960 R. J. COOPER 2,930,746

CONTROL OF REDUCTION POTYLINES Filed Jgn. 10, 1957 v4: Sheets-Sheet :s

INVHVTOR. Fade/7 J COO 06f March 29, 1960 R. J. COOPER 2,930,746

' CONTROL OF REDUCTIQN POT LINES I Filed Jan. 10. 1957 4 Sheets-Sheet 4CONTROL OF REDUCTION POTLINES Robert J. Cooper, Spokane, Wash.Application January 10, 1957, Serial No. 633,436 3' Claims. (Cl.204-225') My invention relates to the reduction of aluminum oxide toaluminum by the process of dissolving the Patented Mar. 29, 19

amount so long as the conditions which affect its re- 1 sistance to thecurrent flow remain most favorabley There must be adequate alumina inthe cryolite bath. 7

There must also not be too much alumina added or it will not dissolveand will settle through the molten aluminum pad. The carbon anode mustbe maintained at the proper distance from the molten aluminum to producethe desired heat and'to decompose the alumina.

I All these things are difiicult to maintain. However, bath aluminumoxide in molten cryolite and passing electric current through thesolution.

In the Well known pot lines whichare commonly used for this purpose, itis the practice to provide a multiplicity of cells or pots which aresubstantially alike, each consisting of a base unit which is Wellinsulated, a carbon shell forming a pan within the 'base and havinginits bottom a grid of metallic conductors. This shell, which is thecathode of the cell, provides a shallow pan in which molten cryolite,molten aluminum and a crust formation on top of the cryolite are held.In normal operation an anode or a plurality of anodes are suspended fromabove the cell to extend into the molten cryolite. Current flows fromthe anode through the molten cryolite in which aluminum oxide isdissolved and decomposes the aluminum oxide. The oxygen thus releasedcombines with the carbon and gradually eats away the carbon anode. Thealuminum metal, being slightly heavier than the cryolite aluminasolution, sinks to the bottom and forms what is usually termed a pad ofmolten metal that increases in depth as the operation proceeds until itis necessary to draw off or tap the pot to reduce the depth of themolten aluminum.

The size of the individual cell or pot is quite large and the amount ofcurrent passing through a series of pots may be of the order of sixtythousand to one hundred thousand amperes. Decomposition potential in theindividual cell is comparatively low, also the electrical resistance inthe cell is considerable so that a major portion of the. potential dropacross a cell is due to electrical resistance of the electrodes, thecryolite bath and the.

electrode connections. It appears to be substantially universal in theindustry to use about five volt potential drop across each pot or cell,thus in a 140 pot line, the total voltage drop across the several potsin series would be 700 volts and the current supply would vary betweensome sixty thousand amperes and one hundred thousand amperes, dependingupon the size of the cells. Since the over all efficiency of the potline is really measured by the amount of aluminum deposited per unit ofpower used, it is obvious that efficient operation of each pot isessential. According to Faradays law, 1,000 amperes will deposit .'74lb. of aluminum per hour at 100% current efiiciency. However, manythings enter into the operation to affect the current efficiency.Current efiiciency in the pots is related directly to bath temperature.It is necessary to maintain the temperature of the bath at about 960degrees C. Much of the electrical resistance in the cell creates heatwhich'operates to maintain this bath temperature. The cell is soinsulated as to maintain the temperature as close as possible to thedesired 960 degrees C.

v In any event theoretically the amount of aluminum made per unit timein a cell is proportional to the current passing through that cell. Acellof a given size will operate efliciently on a current flow of agiven temperature and current flow are both highly responsive tochanging resistance across the electrode space. It is the purpose of myinvention to provide a control means operating automatically to maintainthe current flow through the bath at the optimum level by responding toconditions thatatfect the over all resistance of the cell to currentflow. 7 p

A discussion of those things, apart from fixed resistances, that affectthe. current flow across a cell and their effect on all of the cells ina pot line will, it is believed, facilitate understanding of my controlsystem and how it operates to correct the things that inter fere withthe reduction process.

The overall efficiency of the reduction process is measuredfundamentally in terms of units of aluminum recovered per unit of powerconsumed and units of electrodes used up. There is an optimumtemperature at which the cryolite' bath should remain for bestreduction. This is stated in an article by Reesein Industrial andEngineering Chemistry, vol. 47, No. 10, p. 2069, as about 960 degrees C.Others give the desired temperature of the bath as 970 degrees C. Theheat to maintain'this temperature is' obtained by the current flowthroughthe bath' from the carbon anodes to the molten aluminum pad andthe carbon lining of the cell or pot. The pots are so insulated thatwith normal voltage crop across thepot and normal current flow for thepot, the heat generated is sufficient to maintain this temperature. Anydeviation from the normal affectsf the temperature.

The amount of alumina in, solution in the cryolite at any time affectsthe resistance of the bath and consequently the temperature. Normallythe .bath should contain about 4% but it may vary between 2.5 and 6%with- Q out serious adverse effects. If less than 2.5% dissolved aluminais present in the bath, however, the resistance of the bath may climbrather rapidly. The cause of this climb in resistance has not been fullyexplained.

However, it appears to be accompanied by an increased thickness of gasfilm beneath-the anode, or at least'a' high increase in" the resistanceat the anode-to-bath 1 junction. The molten bath has alimitedcapacity totake alumina, into solution so feeding excess amounts,

of alumina into the bath may result in excess alumina settling throughthe molten cryolite .bath and the molten aluminum beneath the cryoliteupon the lining because the alumina is heavier than'either' the moltencryolite or the molten aluminum. These unreduced particles of aluminaare poor conductorsof electricity;

and tend to increase the overall resistance "of th'e. pot.

Also since the resistance increase is below the already reduced moltenmetal, the amount of alumina decomposed in the cryolite bath may belessened because of lack of adequate potential across the bath eventhough the overall voltage across the pot may be higher than normal.

A pot line electrically resembles a multiplicity of variable resistancesin series with a substantially fixed voltage applied'across them. Sincethey are in series the current flow is the same in all of them. When onepot has any change in resistance it affects the amount of current flowin all the pots. It also affects the voltage drop across the other pots.This in turn may lowerthe power available for reduction in the severalpots below the optimum and result in serious loss of metal production.The desired condition of course, is optimum current flow through theline with each pot having enough voltage drop across it to take care ofthe fixed resistances and leave proper voltage available to decomposealumina and to force current through the resistance of thecryolitealumina solution.

The-voltage necessary to decompose the alumina is some times referred toas the counter-voltage of the pot line. It is of the order of 1.65 to1.75 volts per cell or pot. This voltage does not apparently vary solong as adequate alumina is in the bath regardless of the total voltageexisting across a cell or pot. However, the voltage across a potmay'rise exceptionally high in response to an excess of gas beneath theanodes in a cell. This condition mayresult from too little alumina inthe molten cryolite bath or other causes. This so called anode effectmay be accompanied by a luminous appearance around the anodes caused bycurrent arcing through the gas film between the anode and the bath. Insuch a case the pot is said to be on light. Working in more alumina intothe bath is one aid to getting rid of the anode elfect. This can be doneby breaking in the crust that forms on top of the bath and thus gettingthe alumina in the crust into the bath. Other practices include rakingthe metal in the aluminum pot below the anodes vigorously to causetemporary short circuits from the anode to the aluminum. This cools thegases and disturbs them so they escape more readily. Lowering of theanodes closer to the molten aluminum pad is also used to stop the anodeeifect.

Reference has been made previously to the bad effects of excessivealumina settling through the bath onto the carbon lining. The settledalumina tends to collect around the sides of the lining first and thengradually spreads across the lining beneath the molten metal. The resultis to gradually concentrate the flow of current on the relative freesurface of the carbon lining. Increased heating takes place in the bathdue to the increased over all resistance across the pot. It is wellknown that at temperatures slightly above 1,000 degrees C. the moltenmetal tends to recombine with the released oxygen in the bath. This ofcourse, cuts down the production of metal. To stop the accumulation ofalumina on the bottom of the pot it is a practice to reduce or cut offthe feed of new ore and to bring the anodes closer to the molten metal.These adjustments do keep the temperature down and do aid in convertingalumina in the bath to metal. More important, the reduction inresistance in the sick pot accomplished by bringing the anodes closer tothe molten metal is to reduce the over all resistance of the pot line sothat other pots will get additional current and not be cooled down.

The temperature of a pot is quite critical. If the temperature is toolow the alumina does not go into solution readily and thus tends to passthrough the bath to produce muck deposits beneath the molten metal. Ifthe temperature is too high, the metal tends to reconvert by combiningwith the oxygen in the bath. Cooling of the bath also produces hardercrusts of cryolite at the top of the bath and makes it more difiicult toget alumina into the bath.

It is believed that I have made clear in the foregoing discussion thereflection of the defects in pot operation in three things: the voltageacross an individual pot, the line current through all pots and thetemperature of the individual pots. It is the purpose of my invention topro vide a control system which utilizes changes of voltage drop acrosspots, changes in line current and changes in pot temperature to vary theanode-cathode spacing and thereby maintain greater uniformity in pottemperature and line current.

Another purpose of the invention is to provide a control system which isoperably connected to the anode raising and lowering mechanism of eachindividual pot and which is actuated by line current variation to effectadjustment of anode cathode spacing of all spots and actuated by voltagevariation across individual pots to efiect adjustment of anode cathodespacing of such pots.

Other objects and advantages of my invention will appear from thefollowing description and the accompanying drawings illustrating apreferred form of the invention. It should be understood, however, thatthe drawings and description'are illustrative only and are not intendedto limit the invention except insofar as it is limited by the claims.

In the drawings:

Figure 1 is a cross sectional view through a typical pot of an aluminumreduction pot line;

Figure 2 is a view of the pot taken at right angles to Figure 1 with thelower part of the pot in section and the upper part of the pot in sideview;

Figure 3 is a diagrammatic view illustrating the series circuitarrangement of pots in an aluminum reduction pot line;

Figure 4 is another diagrammatic view illustrating the control system ofmy invention on a plurality of pots;

Figures 5, 6 and 7 are diagrammatic views illustrating the electricalcharacteristics of a pot under three conditions.

In ,a line that has, for example, pots with a voltage of 500 voltsacross all of the pots in series and with a current of 50,000 amperes,the individual pot resistance would be .0001 ohm. Each pot theoreticallywould have a 5 volt drop across it and 50,000 amperes flowing throughit. If we assume that one pot in the pot line gets too little alumina,it heats up suddenly and goes on light.

When a pot is on light, a voltage drop across it of the order of 30volts may be found to exist. The overall voltage rises slightly, if theline voltage regulators are untouched, to about 508 volts. This meansthat other pots will have an average of about 4.83 volts drop acrosseach and the current through all pots will be cut down to 48,300amperes. Obviously the pot on light" is too hot and its efiiciency isdown. The current loss is about 3.4%.

Now assume a pot line of pots with a desired load of 60,000 amperes and700 volts across all of the pots in series. To get the 60,000 amperesthe total resistance across the 140 pots would be .0116 ohm or .0000833ohm per pot. The voltage drop across each pot should be about 4.989volts. If due to either alumina shortage or deposit of undissolvedalumina beneath the molten metal or other causes, the line resistanceincreases by as little as .0013 ohm to .0129 ohm and the voltageregulators are not touched, line voltage will rise to about 710 volts,making an average voltage of 5.065 volts across a single pot. The linecurrent will decrease from 60,000 amperes to 55,000 amperes. The voltagechange is so slight as to be difiicult to detect on the meters commonlyused to indicate pot voltage. However, the 5,000 ampere drop in the lineis an 8.3% loss.

In the case of the pot on light the high voltage across it is easilydetected. In the other case given, the reduction in line current fiow isreadily detected.

The third factor of pot temperature may cause great losses in metalproduction. For example, if the temperature of a pot goes up withcurrent holding steady, then the voltage across this pot should bereduced to avoid the losses in reconversion, etc., that result from toohigh pot temperature. If the pot temperature is running too low we knownthat certain problems arise. For example, the ability of the bath todissolve alumina is reduced. According to my invention, therefore,control means are provided whereby a rise in temperature of a pot caneffect a drop in voltage across the pot and a drop in temperature of apot can etfect a rise in voltage across the pot. In all carbon pots theresistance of the carbon. of the pot is lowered by heating the carbon,because the The temperature range in which a pot must operate is soslight that in the overall pot structure, this characteristic of thecarbon is not critical. Also the metal leads to the .pots have apositive temperature coeificient of resistance which tends to offset thecarbon characteristic. 1 do provide means responsive to the pottemperature as a third control element as will be described later. 7

As illustrated in Figures 1 and 2, an individual pot 9 in a pot line ismade up of a bed 10 of heat insulating material. On thisybed is a pan 11of carbon which forms the cathode of the pot 9. The pan 11 holds amolten metal pad 12 which is formed of the metal reduced from thealumina. Over the pad 12 is the electrolyte or bath 13 which is moltencryolite with certain additives and dissolved alumina. A crust (notshown) forms on the bath 13 and acts as an insulator to keep heat in thebath.

The pot 9 has one or more anodes 14 of carbon suspended by bars 15 fromoverhead frames 16. A hopper 17 is used to supply alumina to the "bathand is usually manually controlled. Ducts 18 are provided for drawingoff gases evolved during the reduction process. Manually removablecovers 19 are provided over the pot. These covers are insulated at theiredges to prevent current flow. Electrical connections to the anodes 14are by means of bus bars 20 clamped against the bars 15 in any suitablemanner that permits individual adjustment of anodes 14. The overheadframes 16 that carry the bars 15 and bus bars 20 are suspended forvertical adjustment.

In the drawings I show the means of suspension as two shafts 21, whichas illustrated best by Figures 2 and 4 are raised and lowered by motors22 and rotatingscrew heads 23 through which threaded portions of theshafts 21 extend. The motors 22 and screw heads 23 are carrled on endframes 24 and 25.v Electrical connection to the cathode pan 11 is byrods 26 embedded in the pan 11 and bus bars 20. It will be understoodthat the bus bars 20 extend from the anode supporting frames 16 of onepot to the cathode pan 11. of the next pot in line since all pots are inseries across the power line as illustrated in Figure 3.

The foregoing description will, it is believed, make the generalconstruction of a pot line for reduction of aluminum sufficiently clearfor the purposes of my invention. It is evident, of course, that atypical pot line may use the pre-baked anodes of the Hall process or theanodes that are baked in place according to the Soderberg process.Without further detailed explanation of the. Well known features of analuminum reduction pot line, the application of my invention theretowill now be described. I might point out that the shaft 21, screw heads23, and motors 22, generally are not used for the purpose of adjustingthe anodes up and down. They do, however, work well for my purpose. Anyequivalent devices may be used for raising and lowering the anodes.

Asv illustrated best in Figure 4 of the drawings, my invention isembodied in a control system that'employs, for each pot, a differentialrelay 27 having one coil 28 connected across the pot between the bus barconnection to the :cathode 11 and the bus bar connection to the anode14. This coil 28 therefore is responsive to fluctuations in the voltagedrop from the anodes 14 to the cathode connected end of the bus bar 20for the pot. The other coil 29 of the relay 27 is connected across avoltage source indicated at 30. The voltage across the coil 29 is variedby a rheostat 31 in response to changes in the amount of current flowingthrough the pots'in series. The rheostat 31 is controlled directly by anammeter which measures the current flowing through the pots. Anexampleof an instrument which can be used for my purpose is a recording ammetersuch as that sold by Leeds '& Northrup Company as a Micromax Recorder"Model S, 4000 series, and described by this company in its directionbook std. 1235. For the purposes of this invention any ammeter issatisfactory which will adjust the'rheostat 31 in response to changes incurrent flow through the pot line to increase the voltage across thecoil '29 when the current flow through the .pot line increases and todecrease the voltage acrossthe coil 29 whenthe current flow through thepot line decreases.

The relay, 27 has its armature 32 biased to neutral position. When thecoil 28 exerts more pull than coil 29, the armature 32 closes its frontcontact 33 and this energizes another relay 34, which closes a point at35 in the energizing circuit for the motor 22. These motors 22 are allreversible motors and the point 35 is in the circuit which causes themotor 22 to rotate in such. a direction as to move the shafts 21 downand lower the anodes 14 toward the cathode 11 of the pot.

When the coil 29 exerts more pull than the coil 28, the armature 32closes its back contact 36. This energizes a relay 37 which closes apoint at 38 in the energizing circuit for the motor 22. The point 38 isin the motor circuit which causes the motor 22 to rotate in thedirection that moves the shafts 21 and anodes 1411p away from thecathode 11. When the current through the pot line is high and thevoltage is normal or low on any pot 9, the anodes 14 of that pot will beraised by energizing its motor 22 through the point 38. However, if, onany pot in the line the voltage is already high, this will offset theeffect of high current and, for this pot, the motor 22 either will notbe energized or will be energized for a shorter period soit will notraise its anodes 14 as much as the other motors 22 will 'raise has aswitch 40 therein and this switch can be shifted to shunt out thecontact (point 35 and to connect the motor 22 directly to a source ofcurrent42 to operate the motor in a direction to lower the anodes 14.Likewise the motor lead to the contact point 38 has. a switch 41 thereinby which the contact point 38 may be shunted out and the motor 22connected directly to the current source 42 to operate the motor 22 in adirection to raise the anodes 14. In the leads to the coil 28 I providea manually variable resistance 43 which can be used to vary the voltageimpressed on the coil 28 for any given voltage drop across a pot. Forexample, it may be desirable to permit a pot to operate on lighttemporarily. By increasing the resistance at 43 sufliciently the effectof the high on light voltage cross the poton the coil 28 can be made toequal the effect of normal pot voltage on this coil. The differentialrelay can be cut entirely out of operation by manually operable switches44 and 45.

In order to utilize the temperature of each individual pot 9 as a factorin its control, a pot temperature respon-' sive device or thermostat 46is positioned in the bath 13 of each pot as indicated in Figure 4. Thisdevice is normally connected in series with the coil 28 of thedifferential relay 27, However, a shunting switch 47 is connected aroundthe'device 46 so that it may be taken the pot 9 in which it is locatedas the temperature of the pot goes up because increased voltage across apot tends to force more current through the pot and thus create moreheatin the pot. In order to have the effect desired, 7 the device 46must reduce the overall resistance in series with the coil 28 as the pot9 heats up from a minimum desirable operating temperature.

In other words, the device 46 must show a'negative temperaturecoeflicient of resistance effect in the circuit of the coil 28. 'Acarbon resistor would have this effect and further has the characteristic of being able to withstand immersion in the bath of the pot.Sensitivity of the carbon, however, is low for this purpose. It must beremembered that permissible temperature variations at the working rangeof a pot are slight in comparison with the temperature at which the potmust be maintained. A bath temperature of 960 degrees C. to 970 degreesC. is desirable, but a rise of the bath temperature to 990 degrees C.causes a significant drop in current efiiciency. I provide a resistanceunit 48 as part of the device 46 with a movable contactor 49 actuated bya temperature responsive element 50 which is extended into the bath 13in a carbon case 51. The temperature responsive element is indicated asa bi-metallic strip. Any equivalent temperature responsive means may beused that is operable to increase the effective pull of the coil 28 onits armature 32 in response to pot temperature rise and to reduce theeffective pull of the coil 23 on its armature 32in response to pottemperature decrease.

It is evident that by giving proper values to the resistance 48, theeffect of temperature rise and fall in a pot 9 upon control of raisingand lowering the anodes 14 may be made such as to maintain pottemperatures within the desired range when the current is kept near theoptimum amount.

As is indicated by Figure 4 of the drawings, each pot 9 has associatedwith it a motorized mechanism consisting of the motors 22, screw heads23 and shafts 21 connected to the anode supports 16 to move the anodes14 toward and away from the cathode 11. The individual anode carbons 14are manually adjustable up and down in the customary manner with respectto other anodes in a pot. The motorized mechanism is actuated by controldevices which are in turn responsive to the current flowing through thepot, the voltage drop across the pot, and the temperature of theelectrolyte bath. The control devices are active conjointly so that forexample, if the voltage is low across a pot and the current is also low,the influence of the low voltage, which is to drive the motorizedmechanism in a direction to separate the anodes further from thecathode, would be opposed by the influence of the low current, which isto drive the motorized mechanism in a direction to bring the anodes 14closer to the cathode 11. The control devices, comprising relays 27, 34and 37 and the thermostat device 46, are so interconnected that thetemperature of a pot, the voltage across the pot, and the currentthrough the pot cooperate to move the anodes 14 with respect to thecathodes.

The action of the control devices may be better seen by reference toFigures 5, 6 and 7 Where the internal elements of a pot are simulated byelectrical resistances. In Figure the normal or desired pot condition ispictured. Here a variable resistance 1 represents the anode, a variableresistance 2 represents the cryolite-alumina bath, a third variableresistance 3 represents the molten aluminum beneath the bath, and afourth variable resistance 4 represents the carbon lining or cathode ofthe pot. In the optimum operation resistance 3 is quite small,resistance 4 is large enough to produce some heat and the anoderesistance 1 is also fairly small. Most of the resistance is in the bathwhich is represented by variable resistance 2.

Assume that in Figure 6 we have an anode efiect and the pot is on light.In this case resistance 1 is now separated from resistance 2 by agaseous film which is represented as a variable resistance 5. The bathresistance increases if the amount of alumina in solution in thecryolite gets too low. This is'what causes anode effects generally, soin Figure 6, resistance 2 is larger than it is in Figure 5. Theresistances 3 and 4 remain essentially the same. It now takes morevoltage to push current through resistance 5 and increased resistance 2and some more heat is produced in the bath. Current in the line will bereduced. All this means that coil 28 of relay 27 will act to energizethe motor 22 on this pot 9 and lower the anodes 4 to reduce both voltageand heat.

Now refer to Figure 7 which illustrates the efiect of undissolvedalumina getting through the bath and depositing on the carbon lining. Inthis condition the resistances 1, 2, 3 and 4 will remain about the same.However, the undissolved alumina inserts an added resistance between theresistances 3 and 4 which resistance is indicated at 6. This resistancewill reduce the current in the line and increase the pot temperature. Itis particularly bad for increasing temperature because the addedresistance is beneath the molten metal where all of the heat generatedis communicated to the bath. The voltage rise may be very slight but thecumulative effect on relay 2? of increased temperature in the pot (whichcuts down the resistance 48 in series with the coil 28, thus effectivelyincreasing the pull of coil 28), and the reduced current through thecoil 29 will cause the energization of the relay 34 to operate the motor22 in a direction to lower the anodes 14. Lowering of the anodes 14reduces the bath resistance 2 and this of course, permits more currentin the line. At the same time, the overall heat generated in the pot isreduced since much of this heat comes from the bath itself.

If it is desired to operate any pot hot for a while, the operator canclose the shunt circuit around unit 46 by moving switch 47 to releasethe control system from the temperature responsive device 46. Areistance 47a is provided in series with the shunt switch 47 to equalthe amount of resistance 48 that would be in circuit with the coil 28 ata pot temperature of 965 degrees C. Thus moving of the switch 47 tosubstitute resistance 47a for resistance 48 frees the automatic controlsfrom the temperature influence.

In the event that it is desired to operate any pot on light" for a shorttime the resistance 43 can be adjusted for this purpose, or if it isdesired to cut off the automatic control entirely from the pot, theswitch 39 is opened or the two switches 40 and 41 can be used to raiseand lower the anodes 14 as desired and left in new tral position. Someoperating plants frown upon the practice of lowering anodes to destroythe gaseous arc in a pot on light. With the present control system it ispossible to provide means whereby the high voltage that is found acrossa pot on light will cut out the automatic regulation of the anodes. Anoverload relay 52 is placed across the leads from the pot 9 to the coil28 and has its armatures 53 movable, when the voltage drop across thepot 9 exceeds a certain predetermined level, to break the power supplycircuit of the motor 22 for that pot.

It is believed that the nature and advantages of my in-- vention will beapparent from the foregoing description. The control system asdescribed, operates to lower the anodes in all pots 9 if the current istoo low and the voltage is normal or too high, unless a pot is too cold.If the pot. temperature is too low the temperature control unit 46 willexert its effect to oppose lowering of the anodes by putting more of theresistance 48 in series with the coil 28 so that less force is exertedby the coil 28 to oppose the coil 29. If the voltage is too high acrossthe pot, this tends to give the coil 28 more force to cause it toovercome the force exerted by the coil 29. Thus a high voltage across apot will tend to lower the anodes 14 by energizing the motor 22 tooperate in the proper direction. Whenever the coil 28 receives enoughenergy to overbalance the coil 29, the anodes 14 are lowered andWhenever the coil 29 receives enough energy to overbalance the coil 28,the anodes 14 are raised. Higher temperature in a pot 9 has the efiectof increasing the energy to the coil 28. Thus under a condition of fullcurrent, normal voltage and normal temperature, the coils 28 and 29 at apot balance. If any pot 9 cools off too much, the energy through thecoil 28 of its relay 27 will be decreased so as to causethe anodes ofthat pot to be raised.

If the voltage across any pot 9 goes up unduly because of shortage ofalumina in the bath, or because of deposit of alumina on the carbon pan,the extra energy pushed through the coil 28 of the control mechanism forthat pot will cause the anodes 14 of that pot to be lowered. If theadded resistance cuts down the overall current in the pot line enough,all of the pots will be affected and the controls thereon will lowertheir anodes. However, cold pots will not be so greatly affected as hotpots so the lowering of the anodes to make up for lost current in theline is selective.

Having thus described my invention, I claim:

1. In a reduction system for metals such as aluminum wherein a pluralityof pots, each embodying a cathode and an anode and each adapted tocontain a molten electrolytic bath into which the compound to be reducedis introduced and dissolved, are interconnected in series to a source ofcurrent, the improvement in control means for said pots comprising powermeans operatively connected to each of the anodes for moving them towardand away from the cathodes, means responsive to the amount of currentflow through the pots and continuously operatively connected to thepower means of each pot tending to cause the power means to move theanodes toward the cathodes upon decrease in current flow through thepots and tending to cause the power means to move the anodes away fromthe cathodes upon increase in current flow through the pots, meansindividual to each pot responsive to variations in voltage across thepot and continuously operatively connected to the power means of thatpot, continuously tending to cause the power means to move the anodecloser to the cathode upon a rise in voltage across the pot and to movethe anode away from the cathode upon a fall in voltage across the pot,and means individual to each pot responsive to variations in thetemperature of said bath and continuously operatively connected to thepower means of that pot, tending to cause the power means to move theanode closer to the cathode upon a rise in temperature of the bath andto move the anode away from the cathode upon a drop in temperature ofthe bath.

2. In a reduction system for metals such as aluminum wherein a pluralityof pots, each embodying a cathode and an anode and each adapted tocontain a molten electrolytic bath connecting said electrodes into whichthe compound to be reduced is introduced and dissolved, are connected inseries to a source of current, the improvement comprising a first meansindividual to each pot responsive to variations in anode to cathodevoltage drop in the pot continuously operable to increase the anode- 101 cathode spacing upon decrease of said voltage drop and to decrease theanode-cathode spacing upon increase of said voltage drop beyondpredetermined limits, a second means individual to each pot responsiveto variations in current flowing through the pot continuously operableto increase the anode-cathode spacing in the pot upon increase in saidcurrent and to decrease the anode-cathode spacing upon decrease incurrent beyond predetermined limits, and means individual to each potresponsive to temperature variations in the pot, said last named beingconnected to one of said first and second means and continuouslyexerting a control eifect thereon tending to cause said one means toincrease the anode-cathode spacing upon temperature decline in the potand to cause said one means to decrease the anode-cathode spacing upontemperature rise in thepot.

3. In a reduction system for metals such as aluminum wherein a pluralityof pots, each embodying a cathode and an anode and each adapted tocontain a molten electrolytic bath connecting said electrodes into whichthe compound to be reduced is introduced and dissolved, are connected inseries to a source of current, the improvement comprising a first meansindividual to each pot responsive to variations in anode to cathodevoltage drop in the pot continuously operable to increase theanode-cathode spacing upon decrease of said voltage drop and to decreasethe anode-cathode spacing upon increase of said voltage drop beyondpredetermined'limits, a second means individual to each pot responsiveto variations in current flowing through the pot continuously operableto increase the anode-cathode spacing in the pot upon increase in saidcurrent and to decrease the anode-cathode spacing upon decrease incurrent beyond predetermined limits and means individual to each potresponsive to temperature variations in the pot, said last named meansbeing connected to said first means and continuously exerting a controleffect thereon tending to cause said first means to increase theanode-cathode spacing upon temperature decline in the pot and to causesaid first means to decrease the anode cathode spacing upon temperaturerise in the pot.

References Cited in the file of this patent UNITED STATES PATENTSFerrand Ian. 17, 1956

1. IN A REDUCTION SYSTEM FOR METALS SUCH AS ALUMINUM WHEREIN A PLURALITYOF POTS, EACH EMBODYING A CATHODE AND AN ANODE AND EACH ADAPTED TOCONTAIN A MOLTEN ELECTROLYTIC BATH INTO WHICH THE COMPOUND TO BE REDUCEDIS INTRODUCED SAID DISSOLVED, ARE INTERCONNECTED IN SERIES TO A SOURCEOF CURRENT, THE IMPROVEMENT IN CONTROL MEANS FOR SAID POTS COMPRISINGPOWER MEANS OPERATIVELY CONNECTED TO EACH OF THE ANODES FOR MOVING THEMTOWARD AND AWAY FROM THE CATHODES, MEANS RESPONSIVE TO THE AMOUNT OFCURRENT FLOW THROUGH THE POTS AND CONTINUOUSLY OPERATIVELY CONNECTED TOTHE POWER MEANS OF EACH POT TENDING TO CAUSE THE POWER MEANS TO MOVE THEANODES TOWARD THE CATHODES UPON DECREASE IN CURRENT FLOW THROUGH THEPOTS AND TENDING THE CAUSE THE POWER MEANS TO MOVE THE ANODES AWAY FROMTHE CATHODES UPON INCREASE IN CURRENT FLOW THROUGH THE POTS, MEANSINDIVIDUAL TO EACH POT RESPONSIVE TO VARIATIONS IN VOLTAGE ACROSS THEPOT AND CONTINUOUSLY OPERATIVELY CONNECTED TO THE POWER MEANS OF THATPOT, CONTINUOUSLY TENDING TO CAUSE THE POWER MEANS TO MOVE THE ANODECLOSER TO THE CATHODE UPON A RISE IN VOLTAGE ACROSS THE POT AND TO MOVETHE ANODE AWAY FROM THE CATHODE UPON A FALL IN VOLTAGE ACROSS THE POT,AND MEANS